Methods for Calibrating Gain E

Implementing Voltage-Reference Trimming

This method of gain calibration can be implemented in two ways: with a manual potentiometer or digital potentiometer.

Initially, using a manual potentiometer may seem to be the simplest method. However, this approach has a disadvantage: automatic calibration is not simple. Instead, a digital potentiometer provides the simplest method of automatic calibration because, in the final test, calibration or even automatic field calibration can be performed easily.

An excellent potentiometer for the example in Figure 4 is the MAX5436, a 128-tap low-drift digital potentiometer with an SPI™-compatible interface. Simply connecting the MAX5436 with no external components gives an adjustment range of typically -5.36% to +6% with a resolution ranging from 0.08% to 0.1%. This range and resolution are more than adequate for most applications.

Conclusions

We examined the problem of gain-error calibration in a data-conversion application. The commonly used digital calibration method was shown to contribute an additional, integral nonlinearity error that is then corrected back. This error also leads to an additional differential nonlinearity error of + or - 1 LSB at the correction point. If this error is not acceptable, then a converter of a higher resolution must be used which, in turn, leads to additional cost.

Calibration can be done by adjusting the voltage reference, either manually or digitally with a digital potentiometer. This avoids the additional DNL and INL errors introduced by the digital method.

Table 1. Maxim's Trimmable References

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